This achievement involved a bit of luck as you will need two opponents together. I got this with the first two handed axe of the knight class, however you can get this with other weapons such as the two handed swords from the knight or vanguard class.
Free-for-alls are best for this as you have more opponents, just run up to two people that are fighting and swing at head level, hopefully you will get the decapitation, if not find two more people together and try again.
Any suggestions for this solution just comment. :D